The present invention relates in general to centrifugal pumps of the pitot tube type, and, more in particular, to an improvement in such pumps that reduces the net positive suction head required to prevent cavitation.
Centrifugal pumps of the pitot type are well known. In general, these pumps include a drive that drives a rotor in rotation within a casing. A pitot pickup in a chamber of the rotor and stationary relative to the rotor intercepts fluid within the chamber and draws that fluid from the chamber. The exiting fluid has a head larger than its inlet fluid head because of energy imparted to the fluid by the rotor. Typically, fluid enters the rotor chamber along a path that includes an annulus surrounding the pitot tube mount and inside the rotor. From this annulus the fluid passes through a plurality of generally radial passages in the rotor to exit near the outer radial limit of the rotor chamber. The pitot inlet in the chamber may be comparatively close to the outer radial limits of the chamber or comparatively close to the axis of rotation of the rotor, depending on the application.
Typically, the pitot tube mount is in the form of a duct or tube extending along the axis of rotation of the rotor and through a wall of the rotor, usually a rotor cover. The duct is attached to the casing, or some other stationary support.
Pitot pumps are noted for their ability to impact large increase in head in the fluid being pumped. Adaptations of these pumps into separators and cleaners are possible because of the opportunity to stratify fluids within the rotor chamber and sort materials according to their density. Stratification, of course, comes from the large centrifugal force field present within the rotor chamber. An example of this application is a separator for separating solids from a liquid. In petroleum applications it is not uncommon to use production fluid from a petroleum well to power downhole machinery. This fluid must be free of solids. A pitot separator separates solids from the power fluid by centrifugal action and removes the solids either through a pitot pickup or nozzles in the wall of the rotor. A second, clean pitot tube pickup draws solid-free material from the chamber. Thus, in this application, it is possible to have more than one pitot pickup within the chamber. Separators, too, can use multiple head pitot pickups, as well as weir-like take-offs from the chamber in the walls of the rotor.
Known pitot pumps include those described in the following U.S. Pat. Nos.: 3,384,024; 3,776,658; 3,795,459; 3,817,659; 3,838,939; 3,926,534; 3,960,319; 3,977,810; and 3,994,618.
In these pitot-type pumps and separators, the duct mounting the pitot tube extends through a wall of the rotor. The duct is stationary while the wall rotates. Fluid inside of the rotor has a considerably higher head than incoming fluid in the annulus on the outside of the duct. Fluid leaking from the rotor chamber into the annulus has a deleterious effect on the net positive suction head of the pump. The net positive suction head (NPSH) is that pressure over and above the vapor pressure of the fluid being pumped within the inlet of the pump required to prevent cavitation in the pump inlet. Cavitation is localized vaporization of fluid. Cavitation adversely affects pump performance by reducing flow rate and discharge head. Cavitation also physically degrades the pump, often quite quickly. In previous designs, the interface between the rotor and the duct provided a labyrinth path for fluid through a plurality of axially spaced, circular grooves on the outside of the duct. Nonetheless, line-of-sight communication between the rotor chamber and the duct above the lands of the grooves and within the bore of the rotor receiving the duct permitted fluid from within the rotor to enter the duct resulting in a high velocity head, even jet-like, with the harmful impact on net positive suction head.